1. Field of the Invention
This invention relates to a photothermographic element and in particular, it relates to a photothermographic element containing pre-formed iridium-doped silver halide grains and preferably pre-formed iridium doped core-shell silver halide grains.
2. Background to the Art
Silver halide-containing photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive material that generates elemental silver when irradiated; (2) a non-photosensitive, reducible silver source; (3) a reducing agent for the non-photosensitive reducible silver source; and (4) a binder. The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and the photosensitive photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathasis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree. ). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image. This produces a black-and-white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
One method of attempting to increase the maximum image density in black-and-white photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating toning agents into the emulsion. Toning agents improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion and producing color images. For example, color images can be formed by incorporation of leuco dyes into the emulsion. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless of very lightly colored. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced as shown, for example in U.S. Pat. Nos. 4,187,108; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
With the increased availability of low-irradiance light sources such as light emitting diodes (LED), cathode ray tubes (CRT), and semi-conductor laser diodes, have come efforts to produce high-speed, photothermographic elements which require shorter exposure times. Such photothermographic systems would find use in, for example, conventional black-and-white or color photothermography, in electronically-generated black-and-white or color hardcopy recording, in graphic arts laser recording, for medical diagnostic laser imaging, in digital color proofing, and in other applications.
Various techniques are typically employed to try and gain higher sensitivity in a photothermographic material. These techniques center around making the silver halide crystals' latent image centers more efficient such as by introducing imperfections into the crystal lattice or by chemical sensitization of the silver halide grains and by improving the sensitivity to particular wavelengths of light by formulating new improved sensitizing dyes or by the use of supersensitizers.
In efforts to make more sensitive photothermographic materials, one of the most difficult parameters to maintain at a very low level is the various types of fog or D.sub.min. Fog is spurious image density which appears in non-imaged areas of the element after development and is often reported in sensitometric results as D.sub.min. Photothermographic emulsions, in a manner similar to photographic emulsions and other light-sensitive systems, tend to suffer from fog.
Traditionally, photothermographic materials have suffered from fog upon coating. The fog level of freshly prepared photothermographic elements will be referred to herein as initial fog or initial D.sub.min.
In addition, the fog level of photothermographic elements often rises as the material is stored, or "ages." This type of fog will be referred to herein as shelf-aging fog. Adding to the difficulty of fog control on shelf-aging is the fact that the developer is incorporated in the photothermographic element. This is not the case in most silver halide photographic systems. A great amount of work has been done to improve the shelf-life characteristics of photothermographic materials.
A third type of fog in photothermographic systems results from the instability of the image after processing. The photoactive silver halide still present in the developed image may continue to catalyze formation of metallic silver (known as "silver print-out") during room light handling or post-processing exposure such as in graphic arts contact frames. Thus, there is a need for post-processing stabilization of photothermographic materials.
Without having acceptable resistance to fog, a commercially useful material is difficult to prepare. Various techniques have been employed to improve sensitivity and maintain resistance to fog.
U.S. Pat. No. 3,839,049 discloses a method of associating pre-formed silver halide grains with an organic silver salt dispersion. U.S. Pat. No. 4,161,408 (Winslow et al.) discloses a method of associating a silver halide emulsion with a silver soap by forming the silver soap in the presence of the silver halide emulsion. No sensitometric benefits for the process of this patent as compared to U.S. Pat. No. 3,839,049 are asserted. The process of U.S. Pat. No. 4,161,408 comprises adding silver halide grains with agitation to a dispersion of a long-chain fatty acid in water, with no alkali or metal salt of said fatty acid present while the acid is maintained above its melting point, then converting the acid to its ammonium or alkali metal salt, cooling the dispersion, and then converting the ammonium or alkali metal salt to a silver salt of the acid.
U.S. Pat. No. 4,212,937 describes the use of a nitrogen-containing organic base in combination with a halogen molecule or an organic haloamide to improve storage stability and sensitivity.
Japanese Patent Kokai 61-129 642, published Jun. 17, 1986, describes the use of halogenated compounds to reduce fog in color-forming photothermographic emulsions. These compounds include acetophenones such as phenyl(.alpha.,.alpha.-dibromobenzyl)ketone.
U.S. Pat. No. 4,152,160 describes the use of carboxylic acids, such as benzoic acids and phthalic acids, in photothermographic elements. These acids are used as antifoggants.
U.S. Pat. No. 3,589,903 describes the use of small amounts of mercuric ion in photothermographic silver halide emulsions to improve speed and aging stability.
U.S. Pat. No. 4,784,939 describes the use of benzoic acid compounds of a defined formula to reduce fog and to improve the storage stability of silver halide photothermographic emulsions. The addition of halogen molecules to the emulsions are also described as improving fog and stability.
U.S. Pat. No. 5,064,753 discloses a thermally-developable, photographic material containing core-shell silver halide grains that contain a total of 4-40 mole % of silver iodide and which have a lower silver iodide content in the shell than in the core. Incorporating silver iodide into the silver halide crystal in amounts greater than 4 mole % is reported to result in increased photosensitivity and reduced D.sub.min. The silver halide itself is the primary component reduced to silver metal during development. The shelf stability properties of the preferred formulations are not addressed. This material is primarily used for color applications.
Japan Patent Kokai 63-300,234, published Dec. 7, 1988, discloses a heat-developable, photosensitive material containing a photosensitive silver halide, a reducing agent, and a binder. The photosensitive silver halide has a silver iodide content of 0.1.about.40 mole % and a core/shell grain structure. The photosensitive silver halide grains are further sensitized with gold. The material is reported to afford constructions with good sensitivity and low fog.
Japan Kokai 62-103,634, published May, 14, 1987; Japan Kokai 62-150,240, published Jul. 4, 1987; and Japan Kokai 62-229,241, published Oct. 8, 1987, describe heat-developable photosensitive materials incorporating core-shell shell grains with an overall iodide content greater that 4 mole %.
U.S. Pat. No. 5,028,523 discloses radiation-sensitive, thermally-developable imaging elements comprising; a photosensitive silver halide; a light-insensitive silver salt oxidizing agent; a reducing agent for silver ion; and an antifoggant or speed enhancing compound comprising hydrobromic acid salts of nitrogen-containing heterocyclic compounds which are further associated with a pair of bromine atoms. These antifoggants are reported to be effective in reducing spurious background image density.
It is well known in the photographic art that when there is an intense level of radiation fluence used during the exposure (such as with flash exposure or such as with a laser scanned exposure), a phenomenon occurs which is referred to in the art as high intensity reciprocity failure (HIRF). The high intensity exposure causes a reduction in the effective speed of the emulsion, it is believed, because the efficiency of the grain's ability to trap photons is reduced and/or there is a solarization effect where the silver halide grains are initially fogged (photoreduced to form metallic silver) by the radiation and then photooxidized by the additional amount of radiation above that needed to form a latent image. This effect has reduced the ability of silver halide emulsions to be used with high power imaging devices.
It has been found that the addition of certain dopants can aid in the reduction of high intensity reciprocity failure. Amongst the more preferred materials known in the art to reduce high intensity reciprocity failure is iridium doping of the silver halide grain. The use of iridium as a dopant for silver halide grains is taught in various different areas of technology. U.S. Pat. No. 4,621,041 teaches the use of iridium dopants in the silver halide component of diffusion transfer printing plates used in conjunction with scanning flash exposures. U.S. Pat. No. 4,288,535 teaches the use of iridium dopants with sulfur sensitizers during chemical ripening to maintain sensitivity and contrast when the emulsions are used with flash exposures (including scanned laser exposure). U.S. Pat. No. 4,173,483 teaches the use of Group VIII metal dopants (including iridium) as a means of reducing reciprocity failure in flash exposed silver halide emulsions. U.S. Pat. No. 4,126,472 teaches the addition of iridium dopants to silver halide grains in combination with hydroxytetrazaindenes and polyoxyethylene compounds. U.S. Pat. No. 4,469,783 discloses the addition of water-soluble iridium compounds to silver halide grains to maintain contrast, even when the grains are subjected to flash exposure. U.S. Pat. No. 4,336,321 discloses the use of iridium as a dopant alone or in combination with rhodium to improve silver halide emulsion performance.
EPO Publication No. 0 569 857 A1 discloses particularly desirable infrared absorbing dyes for use as antihalation dyes in photographic emulsions. The use of iridium dopants in forming the grains, although for no disclosed purpose, is shown.
U.S. Pat. No. 4,828,962 discloses the use of a combination of iridium and ruthenium dopants in silver halide emulsions to reduce high intensity reciprocity failure in photographic elements.
U.S. Pat. No. 4,725,534 discloses the use of metal halide salts to form silver halide on organic silver salts (silver salts of organic fatty acids). The invention emphasizes the growth of the silver halide on the fatty acids in an organic solvent for use in thermally developable photosensitive media (column 3, lines 21-45).
Japanese Patent Publication 90-087 358 discloses the use of iridium dopants in silver halide grains used in heat developable dye forming systems comprising silver halide (with iridium dopants) sensitized to the infrared, dye donative substance, reducing agent and binder.
Japanese Patent Publication Nos. 04-358 144 and 4-348 338 describe the use of iridium dopants in silver halide grains formed in organic solutions. The silver halide grains are then added to silver soaps to form a photothermographic element.
Japanese Patent Application No. 63-300 235 discloses the formation of silver halide grains by the in situ method on silver behenate soaps. The use of Group VIII metals (inclusive of iridium) during the in situ formation is also disclosed.